γ-Cyclodextrin as a capping agent for gold nanoparticles

Cyclodextrins (CDs) are environment-friendly chemicals, which are used as a stabilizing agent for the synthesis of gold nanoparticles (AuNP). However, their binding mechanism and adsorption modes on AuNPs remain not well understood. Herein, the adsorption behavior and favorable binding geometry of gamma-cyclodextrin (gamma-CD) onto the surface of a quasi-spherical AuNP were studied by all-atom molecular dynamics (MD) simulation. The adsorption of gamma-CD onto AuNP was sampled by varying the number of adsorbate molecules from 10 up to 70 and by changing an initial random distribution of adsorbates around AuNP. Our MD simulations reveal that, at the low adsorbate content, gamma-CD molecules prefer binding onto the gold surface by its wider toroid rim so that they occupy all available surface area of AuNP. In contrast, we observed that the increase in the adsorbate content resulted in some steric repulsion among adsorbed molecules and their self-aggregation, so that the preferred adsorption mode of gamma-CD molecules changes in favor of binding by their toroid side. The analysis of radial distribution functions calculated between gold atoms and gamma-CD reveals that anchoring and binding of an adsorbate onto the gold surface occur through multiple non-covalent interactions, among which non-covalent bonding of the primary and secondary hydroxyl groups, as well as the aliphatic carbon moieties play an essential role. Besides, we found that strong binding interactions between AuNP and gamma-CD caused some conformation distortions and twisting of the gamma-CD macrocycle, which was not observed for smaller-ring analogs of alpha-CD and beta-CD, respectively.

Automated summary

The adsorption behavior and favorable binding geometry of gamma-cyclodextrin (gamma-CD) onto the surface of a quasi-spherical AuNP were studied by molecular dynamics simulation. At lower adsorbate content, gamma-CD molecules prefer binding onto the gold surface by its wider toroid rim while at higher content, they change in favor of binding by their toroid side. Binding interactions between AuNP and gamma-CD occur through multiple non-covalent interactions, with hydroxyl groups and aliphatic carbon moieties playing an essential role.